Migration in insects usually occurs prereproductively so the precalling period, between emergence and the first release of pheromone, determines the number of nights over which females of migrants such as Mythimna separata can express their migratory potential. Precalling periods (PCP's) ranged from 3 to 21 nights in F1 females from insects collected in the field in eastern China. As technical problems precluded the use of a sib-analysis design, the genetic basis of this variation was investigated by analysis of the correlation between daughters' and mother's PCP's and by selection in isofemale lines followed by reciprocal crosses. The results of these analyses indicated that PCP is strongly influenced by genes located on the X chromosome, inherited from the male parent in Lepidoptera. Genetic regulation of migratory potential, of which PCP is a crucial component, can be expected to evolve in tropical species such as Mythimna separata, whose larval habitats are associated with unpredictable tropical rainfall. This species has no preimaginal diapause but makes regular seasonal incursions every spring and early summer to high temperate latitudes at which it cannot overwinter. Where migrants are carried by winds, as occurs in these migrations, the predominantly poleward winds at this time of year must create a genetic cline across latitude, in which migratory potential of the summer populations founded by the migrants increases with increasing latitude. We suggest that X-linkage of genes influencing PCP may reduce the genetic load associated with this polymorphism by reducing the frequencies of inappropriate PCP genotypes in these summer populations. Thus, in autumn when winds are predominantly northerly, fewer individuals at any latitude will lack the capability to make the return journey southward. Some support for this model is provided by the observed distributions of PCP's of F1 females from moths collected in the field in central and northern eastern China. If X-linkage of genes influencing PCP contributes to the stability of these migratory cycles, it can be expected in other species with similar migratory strategies.